1. Field of the Invention
The invention relates to a process for monitoring and controlling the quality of rolled products from hot-rolling processes in which rolled products such as sheets, strips, profiles, rods, wire, etc. are produced from input stock such as slabs, thin slabs, blooms, billets, etc.
2. Discussion of Background Information
The input stock solidified in the ingot mold has a fairly coarse primary structure, and the individual crystals have grown inward from the walls in the form of columnar crystals or dendrites. In order to achieve sufficient toughness, the structure must be refined. This is most effectively performed by mechanically breaking up the structure during rolling. The hot forming must be carried out essentially above the upper transformation line in the iron-carbon diagram, approximately in the range from 1100 to 850xc2x0 C., depending on the composition of the steel, the upper temperature range serving mainly for shaping and the lower one for structure refining.
The article xe2x80x9cRechnersimulation der Warmumformung und der Umwandlung am Beispiel der Warmbanderzeugungxe2x80x9d [Computer simulation of hot forming and transformation using hot strip production as an example] from Stahl und Eisen [Steel and Iron] 116 (1996) No. 4 of Apr. 15, 1996, presents the intermeshing models of shaping and structure development while taking into account the local forming characteristics. Individual calculations with various partial models, for example for the kinetics of dissolution and precipitation of microalloy elements and for the recrystallization sequence, underline the efficiency of the system with which complex production tasks can be performed. Thus, causes for the occurrence of an inhomogeneous ferrite structure in a tubular steel are derived from the simulation data and measures for improving the homogeneity are proposed.
The calculation of the effect of the cooling conditions on the transformation behavior of construction steel and the effect of the cooling conditions in the finish-rolled material on the changes in the strength properties over the strip length permit a quantitative evaluation of the influencing variables.
In cold working of the material, free dislocations must first be generated, which can then move along the slip planes. The generation occurs either through pulling off of xe2x80x9canchoredxe2x80x9d dislocations or activation of dislocation sources. Atoms such as C or N attached to dislocations make the pulling off and thus the generation of free dislocations difficult. Macroscopically, the effect is visible by a pronounced yield point. The strain in the material increases until it is sufficient to pull off the dislocation from attached atoms (xe2x80x9cupper yield pointxe2x80x9d), i.e., if there is a sufficient strain, the dislocation is separated from the attached atoms. If the dislocations are then free, less strain is required for further dislocation movement (xe2x80x9clower yield pointxe2x80x9d). The strain necessary for the movement of free dislocations increases again only when impeded by other dislocations.
If the material is cold worked to a small extent before actual processing, dislocations are already generated. The frequency, distribution, and localization of the dislocations generated is affected by the type of cold working (e.g., stretcher-and-roller leveling, temper passing). The resistance which atoms such as C or N oppose the slip along existing dislocation planes is overcome with sufficient cold working (e.g., stretcher-and-roller leveling, temper passing with normal expansions).
FIG. 2 shows schematically a force-strain diagram (force F, strain xcex5) measured in a tensile strength test of a material with a pronounced yield point (Part X), and FIG. 3 shows the force-strain diagram (Part Y) of the same specimen when it was subjected before the tensile strength test to cold working, in this case stretching of xcex51%. FIG. 4 shows part X+Y, which correlates the two aforementioned diagrams of FIGS. 2 and 3. In the tensile strength test with prior deformation, the first part of the force-strain curve of the nondeformed material (X) is blended out. With sufficient prior deformation, the pronounced yield point (the peak in (X)) is also blended out. Depending on the extent of the prior deformation, the yield point can rise or fall relative to the nondeformed material, as long as the nondeformed material has a pronounced yield point. If it has no pronounced yield point, the yield point rises in each case.
To determine the mechanical-technical characteristics Rp0.2 and Rm, the measured forces F0.2 and Fm are divided by the cross-sectional area of the specimen (perpendicular to the direction of tension). In the test (Y), this cross-sectional area is already reduced compared to the test (X). Consequently, in our example in the case (Y), the tensile strength is greater than in the case (X), although the same maximum force Fm was measured.
If sufficient interstitial atoms (C or N) are present in the basic material, they will also diffuse in the cold worked material at room temperature after a more or less longer time period to the dislocations present and pin them. Thus, a pronounced yield point also develops again in the cold worked material under certain circumstances (age hardening). To describe this age hardening, it is above all essential to know the amount of dissolved C and N. In BH-steels (bake hardening steels), this age hardening mechanism is expediently used to obtain a higher yield point after cold working and heat treatment (shortening of the diffusion time).
Only a very small quantity of C can be dissolved in ferrite. With clearly higher C-content, the carbon is precipitated as cementite (Fe3C) in various forms (for example, pearlite, grain boundary cementite, intercrystalline), whereby the respective form and quantity of the cementite precipitates also depend very much on the xcex3-xcex1-phase transformation and the temperature progression. Under ordinary production conditions, in steels with C-content  greater than .20, insufficient carbon to cause age hardening remains dissolved. At a lower C-content, cementite formation occurs more or less completely as a function of the temperature progression, such that sufficient carbon can be dissolved to cause intentional or unintentional age hardening.
In small quantities, carbon can, however, also be bound in precipitates. Above all in steels with very low C-content, alloy elements such as Ti, Nb, V are often used to bind the free carbon by precipitation. In this case, the precise knowledge of the amount and composition of these precipitates is important to calculate the amount of remaining free C.
The nitrogen present in the material can be bound in precipitates with Ti, Nb, Al, among others. Consequently, the precise knowledge of the amount and composition of these precipitates is important to calculate the amount of remaining free N. Above all in steels in which only Al is present as the single significant alloy element for N binding (construction steels and soft steels), under ordinary production conditions in hot-rolled strip production, the cooling curve in the cooling section and in the wound state is significant for AlN formation.
Through knowledge of the amount of dissolved C and N, it is possible to infer a pronounced yield point and thus also the change in the yield point by means of prior cold working. Moreover, it is possible with a cold worked material to calculate the redesigning of a pronounced yield point as a function of the quantity of dissolved C and N and the time elapsed since the cold working as well as the temperature during this time.
The object of the invention is to provide a process with which the material properties of the end product which are to be expected can be calculated in advance at each step of the hot rolling production process.
The above object is achieved by the invention, wherein production conditions such as temperatures, reductions per pass, etc. are detected on-line throughout the entire rolling process, and wherein the mechanical/technological material properties to be expected, in particular the xe2x80x9cyield pointxe2x80x9d, the tensile strength and the breaking elongation point, of the rolled product are calculated in advance therefrom by means of interrelated physical/metallurgical and/or statistical models describing the entire rolling process. On-line detection of the actual and instantaneous production conditions is necessary to enable the material properties to be expected always to be calculated in advance.
In addition, it is advantageous that, in the event of deviations of the precalculated material properties of the rolled product from the required mechanical/technological material properties of said product during the rolling process, the deviations are corrected in the subsequent production step. This ensures that the required mechanical/technological material properties are maintained.
It is furthermore advantageous that the reference chemical analysis of the xe2x80x9cinput stockxe2x80x9d and the production conditions, which are the time-temperature curves and time-temperature deformation curves in the individual production steps, are optimized with the physical/metallurgical and/or statistical models describing the entire rolling process and are established for novel related product qualities. It is therefore possible to establish suitable production conditions for such related product qualities without long test series.
In an embodiment of the process, each input stock is identified and the characteristic properties, such as the chemical analysis, the dimension, the precipitation state arising from the preceding temperature curve, such as, size, amount, type, distribution of precipitates, such as AlN, TIN, TiC, TiNbCN, VC, etc., degree of existing segregations, etc., are input into a physical/metallurgical austenitization and precipitation model which calculates the characteristic material properties, such as austenite grain size and precipitation state, in particular dissolution of precipitates, from the time-temperature curve for heating the input stock to rolling temperature, and the material properties present after the heating, in particular temperature, dimension, austenite grain size and precipitation state, are then input into a physical/metallurgical deformation, recrystallization, transformation and precipitation model which calculates the characteristic properties, in particular austenite grain size, temperature distribution, precipitation state, degree of recrystallization, etc., from the time-temperature deformation sequence during the rolling process, and these material properties are further input into a physical/metallurgical cooling, transformation, precipitation and aging model which calculates the characteristic properties of the rolled product, in particular the microstructure including the proportions of structural constituents, such as austenite, ferrite, pearlite, bainite and martensite, and their properties, such as ferrite grain size, pearlite interlamellar spacing, etc., and the precipitation state, from the cooling curve for the rolled product in a cooling means provided for this purpose and during the subsequent free unforced remaining cooling and aging of the rolled product in the rolled-up, stacked, bundled, etc. state, and the properties describing the rolled product finished for further use, such as dimension, chemical analysis, microstructure and precipitation state, etc., are further input into a physical/metallurgical material model which determines the mechanical/technological material properties of the rolled product taking into account any cold forming, for example stretcher-and-roller leveling. This is a possible detailed sequence of possible steps of the basic process according to the invention.
In a further embodiment of the process, in the event of deviations in the characteristic data of the input stock, of the heating curve, of the rolling curve and of the cooling curve, the changes, necessary for maintaining the required mechanical/technological material properties, in the time-temperature curve for the heating, in the time-temperature deformation curve during rolling and in the time-temperature curve during cooling are calculated on-line and by means of the physical/metallurgical austenitization, deformation, recrystallization, transformation, precipitation, cooling and material models and are transmitted to the control system of the heating, rolling and cooling plant. This ensures the maintenance of the required mechanical/technological material properties of the rolled product within the remaining possibilities.
It is advantageous that the reference chemical analysis of the input stock and production conditions are optimized with the physical/metallurgical austenitization, deformation, recrystallization, transformation, precipitation, cooling and material models and are established for novel related product qualities.
In a further embodiment of the process, statistical models are set up by means of the linear regression method with the data from samples of rolled products and the associated input stock properties and production conditions and are continuously improved with further data from samples of rolled products and the associated input stock properties and production conditions and are adapted to these.
According to one embodiment, an adaptation and an adjustment of the physical/metallurgical models are carried out using the data from rolled products and their input stock properties and production conditions. This ensures that the models are always very close to the actual conditions.
According to a further embodiment, the physical/metallurgical and the statistical models for calculation in advance of the mechanical/technological properties of a rolled product and the on-line correction of the production conditions have been realized on a process computer.
A further advantage is that adaptation, adjustment and improvement of the physical/metallurgical and of the statistical models for calculating in advance the mechanical/technological properties of a rolled product have been realized on a process computer.
It is furthermore advantageous that the physical/metallurgical and the statistical models for optimizing and establishing suitable production conditions for achieving the mechanical/technological properties of a rolled product have been realized on a process computer.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.